From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752790AbcFUQr2 (ORCPT ); Tue, 21 Jun 2016 12:47:28 -0400 Received: from mail-ob0-f178.google.com ([209.85.214.178]:34872 "EHLO mail-ob0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752759AbcFUQrZ (ORCPT ); Tue, 21 Jun 2016 12:47:25 -0400 MIME-Version: 1.0 In-Reply-To: <2575012.qQ3uE1GZEQ@tauon.atsec.com> References: <20160617155952.1021006-1-arnd@arndb.de> <3776557.os2uYrMn6M@wuerfel> <2575012.qQ3uE1GZEQ@tauon.atsec.com> From: John Stultz Date: Tue, 21 Jun 2016 09:47:23 -0700 Message-ID: Subject: Re: [Y2038] [PATCH] crypto: use timespec64 for jent_get_nstime To: Stephan Mueller Cc: Arnd Bergmann , Herbert Xu , Alexander Kuleshov , y2038 Mailman List , lkml , linux-crypto@vger.kernel.org, "David S. Miller" , Kees Cook Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 21, 2016 at 9:34 AM, Stephan Mueller wrote: > Am Dienstag, 21. Juni 2016, 09:22:31 schrieb John Stultz: > > Hi John, > >> On Tue, Jun 21, 2016 at 1:32 AM, Arnd Bergmann wrote: >> > On Tuesday, June 21, 2016 8:20:10 AM CEST Stephan Mueller wrote: >> >> Am Freitag, 17. Juni 2016, 17:59:41 schrieb Arnd Bergmann: >> > Compared to the previous __getnstimeofday(), the difference is >> > >> > - using "monotonic" timebase instead of "real", so the zero time >> > >> > is when the system booted rather than Jan 1 1970 >> >> I haven't looked at the details of the calling code, but I'd worry for >> crypto uses, especially if its being used for entropy collection, >> using the monotonic clock instead of the realtime clock might be >> problematic. > > Funnily it does not seem like that. All tests that I have conducted show that > monotonic clocks behave equally as realtime clocks, because the uncertainty > lies in the execution time of a set of instructions. All we need to do is to > measure it with a timer that has a resolution that allows detecting these > variations. Ok. If you're only using it for interval measurements, then either way shouldn't matter. I just wanted to make sure the entropy wasn't coming from the actual time. >> > - "raw" means we don't honor updates for the rate based on ntp, >> > >> > which is probably better as the ntp state might be observable >> > over the net (it probably doesn't matter, but it can't hurt) >> >> So... this feels like a very vague explanation, and the lack of >> frequency correction here probably need a really good comment. Keeping >> multiple time domains is usually asking for trouble, but we added the >> MONOTONIC_RAW clock to address a few cases where people really wanted >> an abstract hardware counter, which was unaffected by frequency >> corrections. I'd really make sure its clear why this is what you want >> vs the standard system time domain so we don't run into problems >> understanding it later. > > Perfect, that is what I would be interested in. But documenting *why* clearly is the thing I'd very strongly suggest. If we need to make some slight semantic change for whatever reason, I don't want folks worried "we can't do that because the crypto code is using it for voodoo". >> > - "fast" means that in very rare cases, the time might appear >> > >> > to go backwards (it probably can't happen here because you are not >> > called in an NMI). >> >> "fast" really means "safe-for-nmi wrt to locking". The tradeoff being >> that when frequency adjustments occur, and if your code is delayed, >> you might see time go backwards by a small amount. This allows > > My code would not see that as an issue. > >> tracing/sched code (or other code called from NMI) to not have to >> duplicate the timekeeping infrastructure. >> >> I think without a much better explanation, using the "fast" method >> isn't really warranted here. > > Thanks a lot. With that, I would think that the proposed ktime_get_raw_fast_ns > is good for use, which is supported with testing on my system. So.. again, I'd avoid using the "fast" accessor unless there is a clear need or obvious benefit. Which should be documented. thanks -john